Pulses of laser light can make molecules react in ways that are impossible using classical test-tube chemistry: Molecules vibrate, and each molecule has its own “tone,” its own “melody.”

It's a question of finding the right key, and that's something that a “smart” laser beam can do. It can find its way to the right tone.

In a recent issue of Nature it is shown how such a laser can be used to control photosynthesis molecules that gather light.

This is the first time this feat has been done with such large and complicated molecules. Part of the work has been carried out at the Chemistry Center at Lund University in Sweden.

The experimental work has been performed at the Max Planck Institute for Quantum Optics in Garching, Germany, and researchers from the University of Glasgow and Vrije University in Holland have also been involved. The Lund scientist connected with the project is Dr. Jennifer L. Herek. Research has been under way for years in Lund seeking to understand how the process of photosynthesis, when plants transform sunlight and carbon dioxide into energy, works at the molecular level. One aim among others is to be able to utilize an artificial version of photosynthesis in the future production of energy.

“In our experiments we made use of a complex of antenna molecules, pigments that capture light and pass it on to a reaction center. Without all the knowledge gathered in Lund over the years about this complex, that feat would have been impossible. We could have used guesswork, but we would have had only one chance in a million to get it right,” says Jennifer Herek.

Today it’s possible to study extremely rapid chemical processes with the aid of lasers. At the Section for Chemical Physics, for example, scientists can start a chemical reaction with a laser pulse and then send a new pulse that will bounce back with information about what just happened.

This technique has been elaborated by the German research team involved in the project, led by Dr. Marcus Motzkus. It is possible to send several pulses in an extremely short period of time. One pulse registers what is happening; another alters the course of the ongoing reaction.

When the laser gets feedback like this about what it has done, it can adapt its pulses to the result and try to find an optimal pulse, the pulse that can bring about the desired reaction in the molecule. In other words, the laser has become “smart.” It is connected to a computer program containing a so-called evolution algorithm. One pulse after another is generated. The “fittest” ones survive and become the “parent generation” of the next series of pulses. In other words, it’s like biological evolution. The color mix, amplitude, time, and a number of different parameters can be adjusted, and the final result can be a whole series of specifically tailored pulses in a certain order.

In order to show that it is possible to control reactions in complicated molecules, researchers must choose something that can be measured in quantifiable terms. Jennifer Herek explains:

“We have worked with an antenna complex in a purple bacterium that uses photosynthesis. Light is captured by carotenoid molecules and transferred to chlorophyll molecules. On the way, half of the energy is lost. For technical reasons, this time we chose to “hamper” nature rather than to “enhance” it. With the aid of lasers, we tried to obstruct this specific transfer more. It turned out to be 30% less effective. We were also able to show that all we influenced was these particular molecules, and nothing else. After having simplified the effective train of pulses, we could show that this was so by shifting the phase of the electric field of neighboring pulses in the train.”

“For a long time we have nurtured the dream that chemists have of being able to control a reaction without the constraints you have to put up with when you have two or more substances reacting with each other,” says Professor Villy Sundström at the Section for Chemical Physics. “With this new method we can learn even more about how photosynthesis works and ultimately be able to apply this knowledge in the creation of artificial photosynthesis.